Defibrillators are implanted in patients susceptible to cardiac arrhythmias or fibrillation. Such devices provide cardioversion or defibrillation by delivering a high voltage shock to the patient's heart, typically about 500–750V. High voltage capacitors are used in defibrillators to accumulate the high voltage charge following detection of a tachyarrhythmia. In the effort to make implantable devices as small and thin as possible, flat aluminum electrolytic capacitors are used.
Such a flat capacitor is disclosed in U.S. Pat. No. 5,131,388 to Pless et al., which is incorporated herein by reference. Flat capacitors include a plurality of aluminum layers laminarly arranged in a stack. Each layer includes an anode and a cathode, with all of the anode layers and all of the cathode layers being commonly connected to respective connectors. The layers may be cut in nearly any shape, to fit within a similarly shaped aluminum housing designed for a particular application. Normally, the cathode layers are together connected to the housing, while the anodes are together connected to a feed-through post that tightly passes through a hole in the housing, but which is electrically insulated from the housing. The feed-through post serves as an external connector for interfacing with other components.
Flat capacitors may be provided with polymeric housings that eliminate the need for additional insulating layers to insulate conductive layers from the housing, reducing total size and increasing energy density (measured in Joules/cc). Such a housing is disclosed in U.S. patent application Ser. No. 09/130,812, filed Aug. 7, 1998, by inventor D. Carson, which is incorporated herein by reference. This device uses an injection molded two-part plastic “dish-and lid” housing that is ultrasonically welded about its periphery. Electrical feedthrough wires pass from the interior to the exterior through holes provided at the weld line. While effective, this housing requires sidewalls that are wide enough to include mating grooves and ridges for ultrasonic welding. In addition, injection molding requires more than a minimum wall thickness for the major panels to allow molten plastic material to flow through the mold. These thicknesses add to the total capacitor volume, decreasing the energy density from what would otherwise be ideal. In addition, the ultrasonic welding process may be sensitive to out-of-tolerance part dimensions, and requires significant operator care and skill, adding to manufacturing costs.